Good idea! I was searching for this option as well. At this moment it seems not possible to use registerCssFile for a browser specific css and sometimes you just need that. May I suggest to add this third parameter to the registerScriptFile function as well.

how to use conditional classes in <html> tag in case we use a third party library?
ex:
<link rel="stylesheet" href="path/to/font-awesome/css/font-awesome.css">
<!--[if IE 7]>
<link rel="stylesheet" href="path/to/font-awesome/css/font-awesome-ie7.css">
<![endif]-->

We shouldn't merge font-awesome-ie7.css to font-awesome.css because it hard to migrate to new version